This invention relates generally to screens for separating undesirable material from desirable material. More particularly, the present invention relates to screens for screening material used in the production of paper, board, and the like. Such screens are known in various designs and used world-wide in the paper industry.
Screens are known where blades mounted on a rotor have a cross-section shaped, in direction of rotation, similar to the wing of an aircraft and which begins with a bulb-like curve in the direction of rotation, then converges towards the rear and ends in the shape of a narrow droplet. These blades extend in the form of bars and are the same length as the rotor or screen basket. Normally a cylindrical rotor is provided. In the screens known, the surface of the blade facing the screen basket provided for the pulp particle suspension to pass through is curved such that the radial clearance between the function-bearing blade surface and the screen basket surface facing it is reduced first of all to a minimum in the leading sector of the blade. After a brief stretch where the minimum clearance to the facing screen basket surface remains approximately the same, the radial clearance then rises again towards the rear end or rear edge of the blade. The purpose of this is to squeeze the screening material suspension through the openings in the screen basket, assisted by dynamic pressure, in the leading front area of the blade with diminishing clearance to the screen basket. In the rear trailing sector of the blade, whose clearance to the screen basket surface facing it gradually increases, a kind of suction effect is generated on the suspension material already pushed through to the other side of the screen basket in order to achieve a backwashing and rinsing off action of the impurities on the screen basket. This suction effect is known and experts shared the opinion hitherto that blades with a cross-section similar to a narrow, curved droplet have the optimum shape for effectiveness of the rotor and the blades to achieve a substantial backwashing effect and obtain the lowest possible flow resistance as the blade moves through the fibre pulp.
In the present description of the state of the art, it should be added that there are two basic types of screen: There are screens with a screen basket which is fed suspension to be screened from the inside and which have blades inside the screen basket, known as xe2x80x9ccentrifugal screensxe2x80x9d, and there are screens with blades rotating outside the screen basket and brushing over it on the outside with low clearance, where these so-called xe2x80x9ccentripetal screensxe2x80x9d feed the material to be screened to the screen basket from the outside and the cleaned pulp suspension is carried off the inside of the screen basket.
In centripetal screens with screen basket on the inside and blades rotating on the outside of it, the rotating shank of the rotor has a star or disc-shaped support in the inlet or top section of the screen basket, overlapping outwards over this screen basket, and with extensions protruding downwards to which the blades rotating round the screen basket are secured.
The present invention is now based on the observation that the hitherto conventional airfoil shape of the blade surface closer to the screen basket, particularly the planned reduction in clearance between blade and screen basket in the leading sector of the blade is not the optimum design, neither in terms of fluid mechanics and energy efficiency, nor with regard to efficiency of the separation process, flow rates and separation performance.
It was discovered unexpectedly that a substantial improvement can be achieved in the operating results and quality of the cleaned material by modifying the clearance between the surface of the individual blades facing the screen basket and the screen basket surface facing each blade, and by selecting a special shape of blade cross-section.
Thus, the subject of the invention is a screen as described above, where the radial clearance between the leading sector of the blade, viewed in the direction of rotation, particularly the front end or the front edge of the surface of the blade facing the screen basket, and the surface of the screen basket is a minimum clearance and rises to a maximum clearance towards the rearmost sector, or the rear edge, of the blade.
The arrangement of the individual blades according to the invention and the continuous widening in clearance between the screen basket and the surface of the blades succeed in avoiding any pressure build-up. It has been demonstrated that the pressure exerted on the suspension to be screened in order to transport it through the screen is more than adequate to push sufficiently large quantities of pulp suspension through the openings in the screen basket and that no additional increase in pressure by special shaping of the rotor blade leading sector with diminishing clearance to the screen basket surface is required to implement this procedure effectively.
On the contrary, the increased suction effect now applied by the entire blade as a result of the widening clearance to the suction basket surface further back on the blade creates much more effective backwashing and, as a result, better removal of the particles and impurities separated from the screening material from the walls of the screen basket. The blades arranged and shaped according to the invention exert lower pressure or xe2x80x9cunderpressurexe2x80x9d in the pulp suspension over their entire length and span in the direction of rotation, i.e. over the entire surface, compared to the pressure they otherwise apply in the screen casing. As a result, the inverse suction effect on a portion of the pulp suspension that has already passed through the openings in the screen basket is increased and backwashing occurs through the openings in the screen basket. Due to improved removal of the impurities retained, the separation behavior and performance of the screen according to the invention is enhanced. Due to the offset arrangement of the blades there is practically always vorticity at every point of the screen basket, which has the effect of cleaning the screen surface. The liquid flow from the feed may also be influenced favorably with a rotor shaped as a paraboloid of revolution.
The principal advantages of the screen and blades according to the invention are as follows:
Lower energy consumption as a result of the lower pressure build-up in the leading sector of the blade and thus, less flow resistance.
Lower pulsation generated by positioning the narrowest flow cross-section between blade and screw basket walls directly at or in the vicinity of the blade front edge.
Higher turbulence generated at the edges of the individual blades and thus, improved screen exposure for higher throughput and separation performance.
Low pressure shocks towards the screen surface and the area behind it, thus significant improvement in screening quality.
Lower rotor speed at constant throughput and thus, lower energy requirement.
The front or leading sector of the blade forms virtually no or only a small angle with the screen basket surface with minimum clearance to the blade. In those areas where the invention dictates a small angle, this configuration allows a high backwashing rate and the stronger suction effect ensures more effective removal of particle material from the screen basket surface.
The curvature of the blade surface facing the screen basket is important for the desired backwashing and for the suction effect. It has been shown that improved backwashing can be obtained with different curvature at the front and rear sectors, viewed in the direction of rotation, of the individual blades. That is, when the surface of the blades facing the screen basket are more curved in the leading sector than in the trailing sector. Preferably the curvature of the surface of the blade is greater at the front sector by some 5 to 20%, especially by 10 to 15%, than the curvature of the surface of the screen basket and the curvature of the surface of the blade is greater in its trailing sector by some 0 to 9%, preferably by 0 to 4%, than the curvature of the surface of the screen basket, and where preferably the transition zone from the preferably circular cylindrical curvature of the leading sector of the blade to the preferably circular cylindrical curvature of the trailing sector extends over the middle third of the length of the blade, where the transition from the strongly curved, leading sector to the less curved trailing sector takes place steadily. Such design boosts the effectiveness when separating the pulp suspension from the impurity particles.
The choice of different curvatures for the various sectors of the blade surface facing the screen basket allows the blade surface to be adapted effectively to cope with different operating conditions and material compositions. Maintaining a ratio of 0.05 to 0.5, preferably 0.1 to 0.3, between the minimum clearance and the maximum clearance between the surface of a blade and the screen basket also provides an advantage in this connection.
In detailed investigations aimed at optimizing the cross-sectional shape of the rotor blade it was established that the conventional configuration used hitherto with airfoil-shaped cross-section, which is both expensive and relatively complicated to implement technically, is not only unnecessary, but can even hamper effectiveness. Blades of a plate-type design, preferably with the same material thickness from the front edge or tip to the rear edge, provide a simpler and effective blade design which can be manufactured at reasonable cost. The lower centrifugal forces resulting from the plate-type blade cross-sectionxe2x80x94avoiding an airfoil shapexe2x80x94permit a cost-saving, light-weight design, virtually for the first time, and these can also be used with lighter-weight blade holding devices. Advantageously, the blades will have a thickness of 2 to 8 mm, particularly 5 to 6 mm, where preferably the blades are made of curved sheet metal, particularly, with their inner and outer surfaces parallel to one another.
In addition it was found that certain design details are capable of further enhancing the advantageous effects of the present invention. This is true, for example, of the shaping of the front edge of the rotor blade, viewed in direction of rotation, as a narrow rectangle, where the two front, corner edges can be rounded off it necessary.
Various different contours have proved favorable for the shape or form of the blade. For example, the contour of the blades may be narrower in a horizontal projection in their leading front sector than in their rear trailing sector or at the rear edge. The blades may have largely, triangular, deltoid, trapezoidal or dovetailed contours in a horizontal projection. The side edges of the blade may extend in a straight line from a front tip or narrow front edge and diverging at an angle towards the rear, forming an angle of 40 to 120xc2x0, preferably between 60 and 90xc2x0. If the contour is shaped in steps, these steps or indentations need not be located in a straight line by any means, but may describe a concave or convex curve. For the purposes of the invention, the blade contour widening from the front end towards the rear can have edges converging at an angle again in its rearmost third. The blade contour shapes described also contribute towards reducing the flow resistance when the blades move through the pulp suspension.
Furthermore it was established that the suction effect can be further increased if measures are implemented to ensure that the blade surfaces contain strip-like elevations extending at right angles to the direction of rotation of the rotor and on the far side of which an increased suction effect is generated locally in each case in contrast to the conventional suction effect of the blade surface.
The rotor itself can be formed in one piece. It can be expedient to use a design where several individual rotor modules are put together to form a rotor with any desired axial span. The rotor may have several blade supports, for example web plates, extending outwards from its shaft or the rotor body, mounted with uniform clearance to one another. Such features permit a highly robust construction in operation and also mechanical stability.
Additionally the invention includes a blade shaped as a bent or curved plate, preferably with the same material thickness from the front edge or tip to the rear edge. With conventional blades it was considered a disadvantage that these components were solid and heavy structures, particularly because these blades had an airfoil-shaped cross-section. With a blade design of the inventive type it is possible to manufacture a lightweight blade and lend the blades the desired shape by simple means, as well as to adapt the blades to suit different applications.
To facilitate changing the blade, particularly in order to insert blades that have been adapted to suit different rotor speeds or pulp suspensions, the blade may have a base section attached to its inner surface, on which fastening devices are provided. Alternatively, the blade may have screw holes for screwing it to a support.